Nuclear medicine uses radiopharmaceutical products marked by radioactive isotopes emitting gamma radiation for obtaining information on the physiological processes of the human body. The progression of the radioactive products toward an organ or its accumulation in that organ are followed from outside the body by means of a gamma radiation detector, more or less sophisticated, the most common being the scintillation camera or gamma camera of the Anger type. The image obtained by such a camera represents the projection on a reference plane of the three dimensional distribution of the radiopharmaceutical product. A three dimensional image may be obtained by applying the well-known principles of the axial tomography.
Another approach, perhaps less popular but offering many advantages, uses as tracers atoms emitting positrons. The positrons annihilate themselves with electrons and generate two gammas of 511 keV emitted at 180.degree. relatively to each other. By detecting coincidentally these two gammas with two diametrically opposite detectors, the trajectory on which the disintegration has occurred may be determined. By superposing, by means of known techniques of tomographic reconstruction, the multiple trajectories measured by an array of detectors surrounding the source, the distribution of the radioactivity in the volume enclosed by the array of detectors may be derived. The three dimensional image may be obtained by the juxtaposition of two-dimensional images of the radioactivity distribution in adjacent planes, or by direct reconstruction from the multiple inter-plane trajectories.
A typical tomograph comprises an array of individual detectors either separated or not separated, by septa. The detectors may be grouped in the array into one or more rings. The array surrounds the body to be scanned and a suitable electronic circuitry processes the electric signals generated by the detectors so as to obtain the desired image. Typically, the diameter of a detector ring varies from 50 to 100 cm, according to whether the apparatus is adapted for scanning the brain or the entire body. The majority of the existing cameras use (Bi.sub.4 Ge.sub.3 O.sub.12) scintillation detectors (hereinafter "BGO") coupled to photomultiplier tubes. Such cameras have a spatial resolution in the order of one centimeter. Certain models can reach a resolution of 4 to 6 mm FWHM. These resolution values are not the inherent theoretical limits fixed by the position range in tissues and the non-colinearity of emission of annihilation gamma-rays, but rather represent a compromise resulting from physical and technological restrains.
The improvement of the resolution of a tomograph up to three millimeters FWHM, which is close to the theoretical limit, is highly desirable. However, the parallax error which exists in a detector ring has, up to now prevented such improvement out of the region very close to the center of the tomograph.
The parallax error may briefly be defined as the lack of information on the radial position of interaction of a gamma ray in a given detector of the ring. The position of interaction in a detector is a function of probability. In some cases, a gamma ray may pass through a detector without interacting therein and interact in an adjacent detector. Therefore, when a detector generates an output signal, indicating the occurrence of an interaction, the gamma ray may come from anywhere within the channel defined by the projection of the volume of the detector, with a distribution given by the probability of interaction of the gamma in this detector (the so-called "aperture function").
At first sight, a simple way to resolve the parallax problem is to reduce the depth of the detectors to lower the volume of the projection channel to, in turn, reduce the incertitude region and the parallax error. However, a thinner detector implies that more gamma rays will pass throughout without interacting, resulting in a loss of efficiency which may not be acceptable for clinical applications. In a similar manner, the increase of the ring diameter will reduce the parallax error, involving a reduction of efficiency of the device and an increase of the costs due to the larger number of detectors necessary to construct a bigger ring.
An alternative solution which has been adopted in several of the commercially available tomographs consists of inserting septa of a heavy metal (Tungsten, Gold or Uranium) between the detectors to reduce the possibility of a gamma ray passing from one detector into another. To efficiently stop a gamma ray of 511 keV, the septa must be sufficiently thick (more than one mm). However, in a high resolution system where the detectors are typically 3 or 4 millimeters thick, the drop of efficiency of 25 to 50% which would results from the use of such septa, is obviously undesirable.